The present invention relates to radio frequency identification (RFID) tags, and more specifically, to generating location information using multiple transmission frequencies.
Typical RFID tags include an integrated circuit (IC) functionally connected to an antenna. The IC stores unique data for identifying a specific item or user associated with the RFID tag. The IC also modulates a radio frequency (RF) signal that is transmitted or backscattered via the antenna. An external tag reader captures the data signal transmitted by the RFID tag.
RFID tags can be classified as “active” or “passive” devices. Active tags use an internal power source to actively transmit a modulated signal to the tag reader. Passive tags, in contrast, do not actively transmit modulated signals to the tag reader but modulate the electromagnetic waves scattered from the tag's antenna. The signal that is scattered originates from the tag reader. Passive RFID tags are powered by the incident signal transmitted by the tag reader. Passive tags are typically less expensive than active tags but have shorter functional ranges than active RFID tags. RFID tags typically operate in the ultra-high frequency (UHF) or microwave frequency bands.
In some cases, the external tag reader may determine the distance between the reader and the RFID tag. Conventional ranging techniques, such as received signal strength indication (RSSI) for either active or passive RFID tags, work over relatively large areas but suffer from poor accuracy. Time of flight or frequency modulated continuous wave radar may also be used to determine the separation distance but these techniques work poorly for short distance applications because of the difficulties of measuring the small round trip time or frequency delay.